Here are some links that should help you to discover the options out there for plagiarism checkers and hopefully some viable ones via reviews that work for your needs and pocket books. Investigating may be a better option than asking the favorites of colleagues as I suspect many folks only know the checker they or their institutions are using. Also, the free checkers can be tested by doing pastes into them of content you take from books and journal articles you find on the web and seeing how much the checker discovers and pinpoints to the source.